The present invention relates to a method of manufacturing a semiconductor device, wherein a metal or a metal silicide film is used as a gate electrode material.
In a semiconductor integrated circuit device having a MOS transistor, a refractory metal or a refractory metal silicide is used, in some cases, as at least part of the material constituting a gate electrode, thereby to prevent an increase in wiring resistance due to miniaturization. On the other hand, in order to relax concentration of electric field at a lower end portion of the gate electrode of the MOS transistor and to repair defects at end portion of a gate insulating film caused by patterning of the gate electrode and ion implantation for formation of source/drain regions, the lower end portion of the gate electrode and the end portion of the gate insulating film need to be subjected to a heat treatment (so-called "side-wall oxidation") in an oxidizing atmosphere.
In the case where a refractory metal or a refractory metal silicide is used as at least part of the material constituting a gate electrode, if the aforementioned heat treatment is conducted with the refractory metal or refractory metal silicide being exposed, the performance of the device is degraded due to formation of a refractory metal oxide accompanied by degradation of a surface morphology. With reference to the accompanying drawings, a description will now be given of prior art (Japanese Patent Application No. 5-327290) of the means for solving this problem of formation of a refractory metal oxide accompanied by degradation of a surface morphology and effectively supplying an oxidizing agent to the lower end portion of the gate electrode and the end portion of the gate insulating film.
At first, as shown in FIG. 1A, a gate insulating film 102 is formed on a semiconductor substrate 101 by means of thermal oxidation. Then, as shown in FIG. 1B, a conductive polysilicon film 103 is formed by means of LPCVD (Low-Pressure Chemical Vapor Deposition) and ion implantation. Using DC magnetron sputtering techniques, a tungsten silicide film 104 is formed on the conductive polysilicon film 103, as shown in FIG. 1C. Subsequently, as shown in FIG. 1D, a silicon nitride film 105 is formed on the tungsten silicide film 104 by means of LPCVD.
A photoresist is coated on the silicon nitride film 105, and the photoresist film is patterned by means of photoetching. A photoresist pattern 106 is thus formed, as shown in FIG. 1E. Using the photoresist pattern 106 as a mask, the silicon nitride film 105 is patterned by dry etching. The patterned silicon nitride film 105 is then used as a mask, and the tungsten silicide film 104 is selectively removed by dry etching, as shown in FIG. 1F.
A silicon nitride film 107 is deposited on the entire surface of the resultant structure by means LPCVD, as shown in FIG. 1G, and then the silicon nitride film 107 is etched back. Thus, a side wall film 107 of silicon nitride is formed, as shown in FIG. 1H. Using the side wall film 107 and the silicon nitride film 105 on tungsten silicide 104 as masks, the polysilicon film 103 is patterned by means of dry etching, and a structure as shown in FIG. 2 is obtained.
The structure shown in FIG. 2 is subjected to heat treatment in an oxidizing atmosphere, whereby the shape of the lower end portion of the polysilicon film 103 is improved and the defects at the gate end portion of gate insulating film 102 are repaired without causing the oxidation of the high-melting-point silicide film 104 covered with silicon nitride side-wall film 107 and silicon nitride film 105, which is accompanied by degradation of a surface morphology.
Japanese Patent Application No. 6-195810 discloses that a refractory metal film, other than the aforementioned refractory metal silicide film, is used, and a silicon nitride oxide film, a polysilicon film, a single-crystal silicon film or an amorphous silicon film, other than the aforementioned silicon nitride film, is used as the oxidation prevention film.
With this technique, the gate edge characteristics can be improved without causing formation of a refractory metal oxide accompanied by degradation of a surface morphology. However, this method has the following drawbacks:
1) It is necessary to form the oxidation prevention film 105 by means of LPCVD, suspend the patterning of the gate electrode, form the oxidation prevention film 107 by means of LPCVD, and etch back the entire surface of the resultant structure. Thus, the manufacturing cost is increased due to an increase in the number of manufacturing steps, and the time period of manufacture is increased accordingly.
2) Only a transistor, which is as large as double the thickness of the side-wall film 107, as compared to minimum patterning dimensions, can be manufactured. This prevents the integration density and operation speed from being increased.
3) A step of the total-surface etch-back, with which dimensional controllability is low, is required. Thus, the variance in operational characteristics of transistors increases.
4) If the silicon nitride film and silicon films are used as the oxidation prevention film 105 and side-wall film 107, there is a large difference in thermal expansion coefficient between these films and the material of the gate electrode. In addition, a stress is produced by the volume expansion due to oxidation. Consequently, the electrical reliability of the gate insulating film 102 deteriorates, and film removal occurs at the interface between the polysilicon film 103 and the refractory metal film or refractory metal silicide film 104.
5) If boron is used as impurities for providing the polysilicon film 103 with electrical conductivity and if the silicon nitride films are used as oxidation prevention film 105 and side-wall film 107, boron in the polysilicon film 103 penetrates the gate insulating film 102 and diffuses into the semiconductor substrate 101. Consequently, the electrical reliability of the gate insulating film 102 and the operational characteristics of the transistor are degraded.
6) Since the upper surface of the refractory metal film is covered with the non-conductive film, a hole for contact with the gate electrode needs to be formed to such a degree as to expose the upper surface of the refractory metal film. Thereafter, a washing treatment with use of an acid solution cannot be performed. Moreover, an excess amount of etching of the gate electrode film increases in a step of simultaneously forming contact holes for contact with the semiconductor substrate and the gate electrode.